Read-only type optical discs such as CDs (compact discs) and DVDs (digital versatile discs) are widely known as information recording mediums that can record digitized information on copyrighted works such as audio works including pieces of music and video works.
An optical discs such as a CD or and a DVD records digitized information on the contents of one or more than one copyrighted works on the disc-shaped substrate thereof as a pattern of recessed and projecting bits that corresponds to the work or works. Optical discs of the type under consideration are accompanied by a problem from the viewpoint of copyright protection because the data recorded on an optical disc can be reproduced and recorded on a hard disc and then on write once read many type recording mediums to produce so-called casual copies.
Techniques have been proposed to prevent such casual copies by recording copy protection key information on an optical disc and allowing only authorized apparatus that can decode the copy protection key information to reproduce contents data from the optical disc. Known techniques of this type include those disclosed in Jpn. Pat. Appln. Laid-Open Publications Nos. 2004-63008 and 9-128890 (Hei).